In an earlier contribution to this discussion,
wrote:

So in summary my questions a
1) What would be best a vented or unvented mains pressure HWC?

Do you really want either - see below?!

2) Would increasing the header tank by a meter benefit the shower
much? (at present theres only bout 1/2m drop from the tank to the
shower.

No.

3) Will the 15mm gas supply be a problem?

It probably would if you wanted a combi-boiler capable of producing
'real-time' hot water at a decent rate.

4) Will the 15mm mains water in let be a problem?

Very probably. Measure the flow rate at your main inlet point - probably the
kitchen tap. If it's less than 20 litres/minute forget a mains-pressure hot
water solution, be-it combi, non-vented hot cylinder, or thermal store.

5) what rough cost im in Essex.

No idea.

It seems to me that you are overlooking the most obvious - and almost
certainly, cheapest - solution, which is to install a shower pump. It needs
to take its hot feed from the HW cylinder (preferably via an Essex or Surrey
flange, but that may not be necessary) and its cold feed from the cold
header tank. That will give you a decent shower for very little effort.
--
Cheers,
Roger
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